Active element substrate with simplified signal line arrangement having active elements and pixel electrodes and liquid crystal display device using the same

ABSTRACT

An active element substrate and an opposed substrate sandwich a liquid crystal layer to constitute a liquid crystal display device. On the active element substrate, two signal lines respectively charge a pair of pixels that are adjacent to each other in a direction parallel to scanning lines. The two signal lines are provided intensively on a pixel electrode of one of the pair of pixels. In the direction parallel to the scanning lines, a pixel electrode on which signal lines are provided, and a pixel electrode on which signal lines are not provided, are arrayed alternately. This arrangement makes it possible to reduce, while ensuring a wide process margin, the fluctuation of the potential of a terminal to be connected to a pixel electrode (the fluctuation of the potential occurs during OFF-period of an active element due to capacitances respectively provided at superimposed portions of signal lines and a pixel electrode), to simplify the arrangement of signal lines, and to improve the aperture ratio.

This application is a Divisional of application Ser. No. 10/929,441,filed Aug. 31, 2004 now U.S. Pat No. 7,196,745, the entire content ofwhich is hereby incorporated herein by reference in this application.

This nonprovisional application claims priority under 35 U.S.C. § 119(a)on Patent Application No. 310636/2003 filed in Japan on Sep. 2, 2003,and Patent Application No. 203939/2004 filed in Japan on Jul. 9, 2004.The entire contents of these applications are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to an active element substrate in whicheach pixel is provided with an active element (e.g. a thin-filmtransistor, a field-effect transistor, a diode) and a pixel electrode,and relates to a liquid crystal display device using the active elementsubstrate.

BACKGROUND OF THE INVENTION

In the field of liquid crystal display devices, recent trend is thewidespread use of active matrix liquid crystal display devices in whicheach pixel has a nonlinear active element (e.g. a thin-film transistor,a field-effect transistor, a diode). This is because such active matrixliquid crystal display devices can attain excellent image quality byreducing unnecessary signal interferences.

Such active matrix liquid crystal display devices prevent deteriorationof image quality by performing alternating driving. In the alternatingdriving, voltages applied to the liquid crystal layer have alternatelyopposite polarities. This driving method is classified primarily intotwo types: line inversion driving and dot inversion driving. In the lineinversion driving, polarities of the voltages applied to the liquidcrystal layer are alternated on scanning lines. In the dot inversiondriving, polarities of the voltages applied to the liquid crystal layerare alternated on signal lines. The special frequency can be set higherin the dot inversion driving than in the line inversion driving.Therefore, the dot inversion driving can attain more excellent displayquality.

In a liquid crystal device that uses, for example, TFTs (thin-filmtransistors) as active elements, a TFT substrate and an opposedsubstrate are provided, and a liquid crystal layer is sandwichedtherebetween. On the TFT substrate, a plurality of signal lines and aplurality of scanning lines intersect, and a TFT and a pixel electrodeare provided at each intersection. On the opposed substrate, a commonelectrode is provided. Each TFT is connected to a scanning line via agate electrode, to a signal line via a source electrode, and to a pixelelectrode via a drain electrode.

In such a liquid crystal display device, while a TFT is ON, a currentflows from the signal line to the drain, thereby charging a liquidcrystal capacitance Clc, which is provided by the pixel electrode,common electrode, and liquid crystal layer. While the TFT is OFF, thevoltage applied to the liquid crystal capacitance Clc is retained.

Conventionally, the pixel electrode is located in a compartment formedby scanning lines and signal lines. However, according to a recentlyused arrangement, the aperture ratio is increased by superimposing thepixel electrode on the signal lines and scanning lines. To adopt thisarrangement, the pixel electrode is isolated from the signal lines andscanning lines by an interlayer insulating film. FIG. 13( a) illustratesan arrangement in which pixel electrodes 50 are superimposed on signallines 51. In FIG. 13( a), with respect to arbitrary three pixels A, B,and C, an arrangement of pixel electrodes 50A, 50B, and 50C, and signallines 51B and 51C are illustrated. In FIG. 13( a), the pixels A, B, andC are arrayed in the lateral direction, which is parallel to thescanning lines. The signal lines 51B and 51C respectively charge thepixels B and C (to be more accurate, the signal lines 51B and 51C chargethe liquid crystal capacitance Clc). The signal line 51B is connected tothe pixel electrode 50B via the TFT 52B. The signal line 51C isconnected to the pixel electrode 50C via the TFT 52C. In the followingdescription, alphabets (A, B, C, etc.) are omitted from referencenumerals, in the cases where it is not particularly required todiscriminate pixels.

The signal line 51B is provided in such a manner as to bridge theadjacent pixel electrodes 50A and 50B, thereby filling the gaptherebetween. Likewise, the signal line 51C is provided in such a manneras to bridge the adjacent pixel electrodes 50B and 50C, thereby fillingthe gap therebetween.

With this arrangement, in which the pixel electrodes 50 are superimposedon the signal lines 51 and/or on the scanning lines, capacitances areprovided by the signal lines 51 and/or the scanning lines, the pixelelectrodes 50, and superimposed portions of the insulating layer. Amongthe capacitances, particularly of note are capacitances Csd. Thecapacitances Csd are provided by the pixel electrode 50, the signallines 51, and the interlayer insulating film provided therebetween.During an OFF-period of a TFT, there is always a signal flowing on asignal line 51 corresponding to the TFT. The signal is a write signal tobe supplied to a pixel electrode 50 corresponding to a scanning lineother than the scanning line corresponding to the TFT. Therefore, thedrain voltage fluctuates through the capacitances Csd. Accordingly, thevoltage to be retained in the liquid crystal capacitance Clc alsofluctuates. In the case of color display, change of hue is caused if thevoltage to be retained in the liquid crystal capacitance Clc fluctuates.

The dot inversion driving, in which the polarities are alternated on thesignal lines, can effectively reduce the fluctuation of the drainvoltage occurring through the capacitance Csd. In the dot inversiondriving, the polarity of the signal (voltage) applied to a signal line51 is inverted at each horizontal scanning period, which is determinedappropriately. Therefore, there is a 180-degree difference between thephases of adjacent signal lines 51. As a result, although the influenceson the drain potential cannot be eliminated, it is possible to cause theinfluences on the drain potential to be directly opposite, so that theinfluences cancel out each other.

The fluctuation ΔVdr of the drain potential in each pixel is representedby the following formula:ΔVdr=Csd1/Cpix×ΔVs1+Csd2/Cpix×ΔVs2where Csd1 is a capacitance provided by a signal line 51 for chargingthe drain, a pixel electrode 50, and an interlayer insulating film; Csd2is a capacitance provided by an adjacent signal line 51, the pixelelectrode, and the interlayer insulating film; Cpix is a sum ofcapacitances associated with the drain; ΔVs1 is a value of voltagefluctuation obtained by subtracting a pre-change potential of the signalline 51 for charging the drain from a post-change potential of thesignal line 51 for charging the drain; and ΔVs2 is a value of voltagefluctuation obtained by subtracting a pre-change potential of a signalline 51 for charging an adjacent pixel from a post-change potential ofthe signal line 51 for charging the adjacent pixel.

According to FIG. 13( a), the formula can be explained as follows: Csd1is a capacitance provided by the signal line 51B (which charges thedrain of the TFT 52B), the pixel electrode 50B, and the interlayerinsulating film; Csd2 is a capacitance provided by the adjacent signalline 51C, the pixel electrode 50B, and the interlayer insulating film;Cpix is a sum of capacitances associated with the drain; ΔVsl is a valueof voltage fluctuation obtained by subtracting a pre-change potential ofthe signal line 51B (which charges the drain of the TFT 52B) from apost-change potential of the signal line 51B (which charges the drain ofthe TFT 52B); and ΔVs2 is a value of voltage fluctuation obtained bysubtracting a pre-change potential of the adjacent signal line 51C froma post-change potential of the adjacent signal line 51C.

FIG. 15 is a schematic diagram illustrating the fluctuation of the drainpotential of the TFT 52B. The fluctuation is caused by signalsrespectively flowing on the signal lines 51B and 51C. As shown in FIG.15, if ΔVs1 is a positive value, ΔVs2 is a negative value, and viceversa. If the values of Csd1 and Csd2 are equal, and absolute values ofΔVs1 and ΔVs2 are equal, the influences on the drain potential arecompletely cancelled out.

Therefore, in a conventional arrangement, the signal lines 51B and 51Care superimposed on the pixel electrode 50B so that the superimposedareas are identical on the signal line 51B and on the signal line 51C,as shown in FIG. 13( a) and FIG. 14( a) (cross-sectional view of FIG.13( a)).

However, even if the two signal lines 51B and 51C are respectivelyprovided at both edges of the pixel electrode 50B so that thesuperimposed areas are identical on the two signal lines 51B and 51C,the superimposed areas change if the pixel electrode 50B is shifted withrespect to the two signal lines 51B and 51C, as shown in FIG. 13( b) andFIG. 14( b) (cross-sectional view of FIG. 13( b). Therefore, the valuesof Csd1 and Csd2, which are respectively formed at both edges of thepixel electrode 50B, are different. If the values of Csd1 and Csd2 aredifferent, the influences on the drain potential are different. As aresult, ΔVdr differs between a shifted region and a normal region.Therefore, effective values are different between the shifted region andthe normal region. The difference is observed as uneven display.

In order to reduce the change of the values of Csd1 and Csd2 caused bythe shift of the pixel electrode with respect to the signal lines, thefollowing arrangement (ladder structure) is proposed. In FIG. 13( a),the signal line 51B is provided so as to bridge the adjacent signallines 50A and SOB. On the other hand, according to the ladder structure,two branch signal lines 51B-1 and 51B-2 are provided, as shown in FIG.16( a) and FIG. 17( a) (cross-sectional view of FIG. 16( a)). The branchsignal line 51B-1 is located within the region of the pixel electrode50A, and the branch signal line 51B-2 is located within the region ofthe pixel electrode 50B.

FIG. 18 is a plan view illustrating a TFT substrate that adopts theladder structure. In FIG. 18, the plurality of lines extending in thelateral direction are scanning lines 53. The plurality of linesintersecting the scanning lines 53 are signal lines 51. Indicated byvirtual lines, pixel electrodes 50 are provided so as to be superimposedat peripheral portions on the scanning lines 53 and signal lines 51. Ona light transmitting substrate (not shown) made of such material asglass, the scanning lines 53, the signal lines 51, and the pixelelectrodes 50 are provided in this order. Between an electrode layerincluding the scanning lines 53 and an electrode layer including thesignal lines 51, a gate insulating film (not shown) is provided. Betweenthe electrode layer including the signal lines 51 and an electrode layerincluding the pixel electrodes 50, an interlayer insulating film (notshown) is provided.

Here, attention is focused on the pixel B. The signal line 51B isdivided into the branch signal lines 51B-1 and 51B-2, except the portionat which the TFT 52B (hatching part) is located. The branch signal lines51B-1 and 51B-2 are provided within the regions of the two adjacentpixel electrodes 50A and 50B, respectively. In FIG. 18, the referencenumeral 55 indicates a Cs wire, and the reference numeral 54 indicates aCs electrode. The Cs wire and the Cs electrode provide a storagecapacitance. The Cs wire 55 is provided in the same electrode layer withthe scanning lines 53, and the Cs electrode 54 is provided in the sameelectrode layer with the signal lines 51.

According to this structure, if there is an enough distance between twobranch signal lines 51-1 and 51-2, the superimposed areas of the pixelelectrode 50B and the branch signal lines 51B-2 and 51C-1, which arerespectively provided at both edges of the pixel electrode 50B, do notchange (see FIG. 16( b) and FIG. 17( b)), even if the pixel electrode 50is shifted with respect to the signal lines 51. Thus, the differencebetween the values of Csd1 and Csd2 can be reduced. As a result, it ispossible to set a wide process margin (see, for example, JapanesePublication for Unexamined Patent Application, Tokukaihei No. 09-152625(publication date: Jun. 10, 1997; corresponding to U.S. Pat. No.6,052,162, No. 5,953,084, No. 6,097,452, No. 6,195,138B1, and No.6,433,851B2), and Japanese Publication for Unexamined PatentApplication, Tokukaihei No. 10-253988 (publication date: Sep. 25,1998)).

The explanation above uses TFTs as one example of active elements, theinfluences of parasitic capacitances formed by the pixel electrode andthe signal lines are described as a fluctuation of potential of thedrain (drain terminal). The same holds true with other active elements,such as field-effect transistors and diodes. That is, the potential ofthe terminal connected to the pixel electrode fluctuates due to theinfluences of the parasitic capacitances.

In general, a color-display liquid crystal display device has colorfilters of three primary colors (red, blue, and green). The colorfilters of three primary colors constitute a block, and the blocks arearranged in a mosaic-like shape or in strips. Other than such a liquidcrystal display device having color filters of odd-number cycle, thereis a liquid crystal display device having color filters of even-numbercycle, in which the four kinds of filters (red, blue, green, and, inaddition, white) constitute a block, and the blocks are arrayed inmatrix. An example of the liquid crystal display device having colorfilters of even-number cycle is disclosed in Japanese Publication forUnexamined Patent Publication, Tokukaihei 02-118521 (publication date:May 2, 1990). With this arrangement, the use of the white filterincreases the overall brightness.

However, the ladder structure is inevitably complex, because a signalline 51 of each pixel is divided into two. Moreover, because the twosignal lines 51-1 and 51-2 are provided with respect to each pixelelectrode 50, the areas occupied by the signal lines 51 is increasedwith respect to the areas of apertures. As a result, the aperture ratiodecreases.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an active elementsubstrate and a liquid crystal display device that can (i) reduce, whileensuring a wide process margin, the fluctuation of the potential of aterminal to be connected to a pixel electrode (the fluctuation of thepotential occurs during OFF-period of an active element due tocapacitances respectively provided by superimposing a pixel electrode onsignal lines), (ii) simplify the arrangement of signal lines, and (iii)improve the aperture ratio.

To attain the foregoing object, a first active element substrate of thepresent invention includes a plurality of signal lines; a plurality ofscanning lines intersecting the signal lines; an active element providedat each intersection between the signal lines and the scanning lines;and a pixel electrode provided at each intersection between the signallines and the scanning lines, the pixel electrode being superimposed atleast on the signal lines, wherein: signal lines respectivelycorresponding to a pair of pixel electrodes are provided intensively onone of the pair of pixel electrodes so as to be located between edges ofsaid one of the pair of pixel electrodes, the pair of pixel electrodesbeing adjacent to each other in a direction parallel to the scanninglines.

Note that a signal line corresponding to a pixel electrode means asignal line that charges a pixel including the pixel electrode. From theside of the signal line, the pixel electrode charged by the signal linecan be expressed as a corresponding pixel electrode. Also not that,although such expressions as “signal lines are provided on a pixelelectrode” is used in order to describe the superimposition of thesignal lines and pixel electrode, this does not mean a hierarchicalrelationship on the substrate among the signal lines, scanning lines,and pixel electrodes. To provide easy-to-understand explanation, TFTsare used as an example of active elements, and effects of the presentinvention are described using the phenomena caused in the case of TFTs.

According to this arrangement, the signal lines respectivelycorresponding to the pair of pixel electrodes are provided only on oneof the pair of pixel electrodes. Therefore, on a pixel electrode, twosignal lines (including a signal line corresponding to an adjacent pixelelectrode) are provided, or no signal line is provided at all.

If such an active element substrate is used in the liquid crystaldisplay device that is driven by the dot inversion driving, in a pixelof a pixel electrode on which two signal lines are provided,fluctuations of the drain (an example of the terminal connected to thepixel electrode) potential occurs in opposite directions, thefluctuation occurring during an OFF-period of a thin-film transistor (anexample of an active element) through the capacitances Csd1 and Csd2respectively provided at superimposed portions. Therefore, as in theladder structure described above, it is possible to reduce thefluctuation of the drain potential caused by the capacitances Csd (Csd1and Csd2), thereby improving display quality.

In addition, the two signal lines provided on a pixel electrode arelocated between edges of the pixel electrode, the edges being parallelto the signal lines. Therefore, even if the pixel electrode is shiftedwith respect to the signal lines, the area of the superimposed portionsof the signal lines and the pixel electrode does not change. As aresult, the capacitances provided at the positions where the two signallines are respectively provided are low. This makes it possible to set awide process margin.

On the other hand, in a pixel in which no signal line is provided on apixel electrode, capacitances are respectively provided by obliqueelectric fields between the signal lines and the pixel electrode, ifsignal lines are provided in adjacent pixels. However, the two signallines that respectively provide the capacitances by means of the obliqueelectric fields are supplied with signals of opposite polarities,respectively. Therefore, the influences exerted through the capacitanceson the drain potential cancel out each other.

Even if the pixel electrode is shifted with respect to the signal lines,the influence of the shift on the capacitances respectively provided bythe oblique electric fields are too small to be a problem. This isbecause the pixel electrode and the signal lines are distanced (notsuperimposed).

Such an arrangement is simpler than the conventional ladder structure,because the signal lines of each pixel are not divided. In addition, theaperture ratio of the panel as a whole is improved, because the areaoccupied by the signal lines with respect to the area of apertures isreduced. This is particularly suitable for a high-definition liquidcrystal display device having a short pixel pitch.

Therefore, there is an effect that it is possible to provide an activeelement substrate that can (i) reduce, while ensuring a wide processmargin, the fluctuation of the potential of a terminal to be connectedto a pixel electrode (the fluctuation of the potential occurs duringOFF-period of an active element due to capacitances respectivelyprovided by superimposing a pixel electrode on signal lines), (ii)simplify the arrangement of signal lines, and (iii) improve the apertureratio.

To attain the foregoing object, a second active element substrate of thepresent invention includes a plurality of signal lines; a plurality ofscanning lines intersecting the signal lines; an active element providedat each intersection between the signal lines and the scanning lines;and a pixel electrode provided at each intersection between the signallines and the scanning lines, the pixel electrode being superimposed atleast on the signal lines, wherein: each of the signal lines includes aportion provided on a corresponding pixel electrode and a roundaboutportion provided on a pixel electrode that is adjacent, in a directionparallel to the scanning lines, to said corresponding pixel electrode;and, on each pixel electrode, a portion of a corresponding signal lineand a roundabout portion of a signal line corresponding to an adjacentpixel electrode form a pair, and the pair is located between edges ofthe pixel electrode except those portions that lie between pixelelectrodes.

Again, a signal line corresponding to a pixel electrode means a signalline that charges a pixel including the pixel electrode. From the sideof the signal line, the pixel electrode charged by the signal line canbe expressed as a corresponding pixel electrode. Also not that, althoughsuch expressions as “signal lines are provided on a pixel electrode” isused in order to describe the superimposition of the signal lines andpixel electrode, this does not mean a hierarchical relationship on thesubstrate among the signal lines, scanning lines, and pixel electrodes.To provide easy-to-understand explanation, TFTs are used as an exampleof active elements, and effects of the present invention are describedusing the phenomena caused in the case of TFTs.

According to the foregoing arrangement, each signal line includes aroundabout portion, and, on each pixel electrode, a part of acorresponding signal line and a roundabout part of an adjacent signalline are provided as a pair. Therefore, in this case, each pixelelectrode includes a portion on which two signal lines are provided(including a signal line corresponding to an adjacent pixel electrode)and a portion on which no signal line is provided at all. Moreover, thetwo signal lines are located between edges of the pixel electrode,except those portions that lie between pixel electrodes. Therefore, thesuperimposed area of the signal lines and the pixel electrode does notchange, even if the pixel electrode is shifted with respect to thesignal lines. As a result, it is possible to reduce the change of valuesof capacitances provided where partial signal lines are provided as apair.

Like the first active element substrate described above, if such anactive element substrate is used, for example, in the liquid crystaldisplay device that is driven by the dot inversion driving, fluctuationsof the drain (an example of the terminal connected to the pixelelectrode) potential occurring in each pixel during an OFF-period of athin-film transistor (an example of an active element) through thecapacitances Csd1 and Csd2 respectively provided at superimposedportions can be reduced as in the ladder structure described above,while ensuring a wide process margin. Therefore, display quality isimproved. Moreover, the arrangement is simpler as compared with theconventional ladder structure, because the signal lines of each pixelare not divided. In addition, the area occupied by the signal lines withrespect to aperture portions is reduced. As a result, the aperture ratioof the panel as a whole is improved. In particular, the forgoingarrangement is advantageous in that the signal lines are allocated toeach pixel evenly in terms of the area occupied by the signal lines.

Therefore, as in the case of the first active element substrate, thereis an effect that it is possible to provide an active element substratethat can (i) reduce, while ensuring a wide process margin, thefluctuation of the potential of a terminal to be connected to a pixelelectrode (the fluctuation of the potential occurs during OFF-period ofan active element due to capacitances respectively provided bysuperimposing a pixel electrode on signal lines), (ii) simplify thearrangement of signal lines, and (iii) improve the aperture ratio.

To attain the foregoing object, a first liquid crystal display device ofthe present invention includes: a first active element substrateincluding a plurality of signal lines, a plurality of scanning linesintersecting the signal lines, an active element provided at eachintersection between the signal lines and the scanning lines, and apixel electrode provided at each intersection between the signal linesand the scanning lines, the pixel electrode being superimposed at leaston the signal lines, wherein signal lines respectively corresponding toa pair of pixel electrodes are provided intensively on one of the pairof pixel electrodes so as to be located between edges of said one of thepair of pixel electrodes, the pair of pixel electrodes being adjacent toeach other in a direction parallel to the scanning lines; an opposedsubstrate on which a common electrode is provided; and a liquid crystallayer sandwiched between the active element substrate and the opposedsubstrate, a polarity of a voltage applied to one of two adjacent signallines being opposite a polarity of a voltage applied to the other of thetwo adjacent signal lines.

To attain the foregoing object, a second liquid crystal display deviceof the present invention includes: a second active element substrateincluding a plurality of signal lines, a plurality of scanning linesintersecting the signal lines, an active element provided at eachintersection between the signal lines and the scanning lines, and apixel electrode provided at each intersection between the signal linesand the scanning lines, the pixel electrode being superimposed at leaston the signal lines, wherein each of the signal lines includes a portionprovided on a corresponding pixel electrode and a roundabout portionprovided on a pixel electrode that is adjacent, in a direction parallelto the scanning lines, to said corresponding pixel electrode, andwherein, on each pixel electrode, a portion of a corresponding signalline and a roundabout portion of a signal line corresponding to anadjacent pixel electrode form a pair, and the pair is located betweenedges of the pixel electrode except those portions that lie betweenpixel electrodes; an opposed substrate on which a common electrode isprovided; and a liquid crystal layer sandwiched between the activeelement substrate and the opposed substrate, a polarity of a voltageapplied to one of two adjacent signal lines being opposite a polarity ofa voltage applied to the other of the two adjacent signal lines.

With the foregoing arrangement, as described above, there is an effectthat it is possible to provide a liquid crystal display device that can(i) reduce, while ensuring a wide process margin, the fluctuation of thepotential of a terminal to be connected to a pixel electrode (thefluctuation of the potential occurs during OFF-period of an activeelement due to capacitances respectively provided by superimposing apixel electrode on signal lines), (ii) simplify the arrangement ofsignal lines, and (iii) improve the aperture ratio.

Furthermore, the first liquid crystal display device of the presentinvention may be arranged so that a pixel electrode on which signallines are provided and a pixel electrode on which no signal line isprovided are arrayed alternately in the direction parallel to thescanning lines; an aperture area of a pixel of a pixel electrode onwhich no signal line is provided is one-half of an aperture area of apixel of a pixel electrode on which signal lines are providedintensively; and color filters of red, green, and blue are provided sothat four adjacent pixel electrodes arrayed in the direction parallel tothe scanning lines constitute one color unit, the four adjacent pixelelectrodes consisting of two pixel electrodes on which signal lines areprovided, and two pixel electrodes on which no signal line is provided,the two pixel electrodes on which no signal line is providedrespectively corresponding to color filters of green.

According to this arrangement, the number of green pixels, which have ahigh level of visibility for human, is twice the number of red and bluepixels. Therefore, higher resolution can be attained as compared with aliquid crystal display device in which the same number of pixels areprovided for each color. In addition, because a green filter is providedso as to be opposed to a pixel electrode on which no signal line isprovided, the total aperture area is the same among the pixels of red,blue, and green, even if the number of green pixels is twice the numberof red and blue pixels. As a result, an excellent white balance isattained.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is an explanatory diagram schematically illustrating anarrangement of pixel electrodes and signal lines on a TFT substrate of afirst embodiment of the present invention. FIG. 1( b) is an explanatorydiagram schematically illustrating an arrangement of pixel electrodesand signal lines on a TFT substrate of a conventional ladder structure.

FIG. 2 is an explanatory diagram schematically illustrating anarrangement of a TN-mode liquid crystal display device using a TFTsubstrate of one of the embodiments of the present invention.

FIG. 3 is an explanatory diagram schematically illustrating anarrangement of an MVA-mode liquid crystal display device using a TFTsubstrate of one of the embodiments of the present invention.

FIG. 4 is a plan view schematically illustrating a structure of the TFTsubstrate of the first embodiment of the present invention.

FIGS. 5( a) and 5(b) are explanatory diagrams illustrating capacitancesCsd provided in a pixel on which signal lines are provided intensively,in a liquid crystal display device using the TFT substrate of FIG. 4.

FIGS. 6( a) and 6(b) are explanatory diagrams illustrating capacitancesCsd provided in a pixel on which signal lines are provided intensively,in the liquid crystal display device using the TFT substrate of FIG. 4.FIG. 6( a) is a cross-sectional view corresponding to FIG. 5( a), andFIG. 6( b) is a cross-sectional view corresponding to FIG. 5 (b).

FIGS. 7( a) and 7(b) are explanatory diagrams illustrating capacitancesCsd provided in pixels on which no signal line is provided, in theliquid crystal display device using the TFT substrate of FIG. 4.

FIGS. 8( a) and 8(b) are explanatory diagrams illustrating capacitancesCsd provided in pixels on which no signal line is provided, in theliquid crystal display device using the TFT substrate of FIG. 4. FIG. 8(a) is a cross-sectional view corresponding to FIG. 7( a), and FIG. 8( b)is a cross-sectional view corresponding to FIG. 7( b).

FIG. 9 is a plan view schematically illustrating a structure of a TFTsubstrate of a second embodiment of the present invention.

FIG. 10 is a plan view schematically illustrating a structure of a TFTsubstrate of a third embodiment of the present invention.

FIG. 11 is a diagram illustrating a color arrangement of a color filerprovided to a liquid crystal display device using a TFT substrate of afourth embodiment of the present invention.

FIG. 12 is a schematic plan view of a liquid crystal display deviceincluding the TFT substrate of the fourth embodiment of the presentinvention and the color filter of FIG. 11.

FIGS. 13( a) and 13(b) are explanatory diagrams illustratingcapacitances Csd provided in a pixel, in a liquid crystal display deviceusing a conventional TFT substrate.

FIGS. 14( a) and 14(b) are explanatory diagrams illustratingcapacitances Csd provided in a pixel, in the liquid crystal displaydevice using the conventional TFT substrate. FIG. 14( a) is across-sectional view corresponding to FIG. 13( a), and FIG. 14( b) is across-sectional view corresponding to FIG. 13( b).

FIG. 15 is an explanatory diagram schematically illustratingfluctuations of a drain potential, the fluctuations being caused by asignal flowing on a signal line.

FIGS. 16( a) and 16(b) are explanatory diagrams illustratingcapacitances Csd provided in pixels, in a liquid crystal display deviceusing another conventional TFT substrate (ladder structure).

FIGS. 17( a) and 17(b) are explanatory diagrams illustratingcapacitances Csd provided in pixels, in the liquid crystal displaydevice using another conventional TFT substrate (ladder structure). FIG.17( a) is a cross-sectional view corresponding to FIG. 16( a), and FIG.17( b) is a cross-sectional view corresponding to FIG. 16( b).

FIG. 18 is a plan view schematically illustrating a structure of a TFTsubstrate in which the conventional ladder structure is adopted.

DESCRIPTION OF THE EMBODIMENTS

With reference to FIGS. 1 to 12, the following describes one embodimentof the present invention.

Discussed below is a case in which an active element substrate is usedin a liquid crystal display device. However, the active elementsubstrate can be used in other display devices, such as anelectroluminescence display device. Moreover, the active elementsubstrate can be used in a light-receiving device that stores charge inpixel electrodes, the charge being generated by radiation of light. Anexample of such a light-receiving device is an X-ray device thatreceives X-ray. Although the active elements used below are TFTs(thin-film transistors), other active elements (e.g. field-effecttransistors, diodes) may be used instead of the TFTs. This is because,as described above, the parasitic capacitances provided by the pixelelectrode and signal lines have similar influences, even if other activeelements are used.

A TFT substrate (active element substrate) 1 of the followingembodiments is one member for sandwiching a liquid crystal layer 3, theother member being an opposed substrate 2. In this way, liquid crystalcells are formed. The liquid crystal cells are sandwiched between a pairof polarizing plates 9 and 10. Thus provided is a liquid crystal displaydevice 20.

The opposed substrate 2 includes a light-transmitting substrate 6, acolor filter 7, and a common electrode 8. The light-transmittingsubstrate 6 is made of such material as glass. The color filter 7 andthe common electrode 8 are formed in this order on thelight-transmitting substrate 6. On a light-transmitting substrate 4, theTFT substrate 1 (fully described later) has a plurality of signal lines(not shown), a plurality of scanning lines (not shown), TFTs (activeelements; not shown), and pixel electrodes 5. The light-transmittingsubstrate 4 is made of such material as glass. The plurality of scanninglines intersect the plurality of signal lines. The TFTs are respectivelyprovided at intersections between the signal lines and scanning lines.

The liquid crystal layer 3 is made of nematic liquid crystal materialhaving a positive dielectric anisotropy, for example. The long axis ofeach liquid crystal molecule in the liquid crystal material issubstantially parallel to the surfaces of the substrates 4 and 6.Between the upper substrate 6 and the lower substrate 4, the liquidcrystal molecules are twisted continually by 90 degrees (twistedalignment). With this liquid crystal layer 3, TN (twisted nematic) modecells in TN mode can be formed.

In the TN mode liquid crystal display device 20, when no voltage isapplied, cells (leftmost pixel and middle pixel in FIG. 2) emit incidentlinear polarized light after changing the polarization direction of theincident linear polarized light by 90 degrees, by the rotatory power ofthe cells. On the other hand, when a voltage is applied, the cells(rightmost pixel in FIG. 2) emit the incident linear polarized lightwithout changing the polarization direction. Therefore, if (i) thepolarization axis of the polarizing plate (one of the pair of polarizingplates 9 and 10) that is closer to the incident side is parallel to thelong axis of each liquid crystal molecule, and (ii) the polarizationaxis of the polarizing plate (one of the pair of polarizing plates 9 and10) that is closer to the emission side is perpendicular to the longaxis of each liquid crystal molecule, bright display is performed whileno voltage is applied, and dark display is performed while a voltage isapplied. On the other hand, if the polarization axes of both thepolarizing plates 9 and 10 are parallel to the long axis of each liquidcrystal molecule, dark display is performed while no voltage is applied,and bright display is performed while a voltage is applied.

Alternatively, the following arrangement may be adopted, for example,thereby forming a liquid crystal display device 21. Namely, the liquidcrystal layer 3 is made of nematic liquid crystal material having anegative dielectric anisotropy. The long axis of each liquid crystalmolecule in the liquid crystal material is substantially vertical to thesurfaces of the substrates 4 and 6. As shown in FIG. 3, specialprotruding patterns 11 are provided on the common electrode 8 of theopposed electrode 2 and on the pixel electrodes 5 of the TFT substrate1. The patterns 11 on the opposed substrate 2 and the patterns 11 on theTFT substrate 1 are provided so as not to be opposed to each other. Thisarrangement allows for providing an MVA mode liquid crystal displaydevice having a wide viewing range characteristic.

To prevent the deterioration of image quality, dot inversion driving isperformed in the liquid crystal display devices 20 and 21. In the dotinversion driving, the polarities of voltages applied to the liquidcrystal layer are alternated on the signal lines.

Discussed below is a substrate structure of the TFT substrate 1 adoptedin the liquid crystal devices 20 and 21.

First, with reference to FIGS. 1( a) and 1(b), the substrate structureof the TFT substrate 1 of a first embodiment is described. FIG. 1( a)illustrates how (i) pixel electrodes 5A, 5B, 5C, and 5D of arbitraryfour pixels A, B, C, and D, and (ii) signal lines 12A, 12B, 12C, and 12Dfor respectively charging the pixels A to D are arranged on the TFTsubstrate 1. In FIG. 1( a), the pixels A to D are arrayed in the lateraldirection, which is parallel to the scanning lines. For the purpose ofcomparison, FIG. 1( b) illustrates how (i) pixel electrodes 50A, 50B,50C, and 50D of arbitrary four pixels A, B, C, and D and (ii) signallines 51A, 51B, 51C, and 51D for respectively charging the pixels A to Dare arranged on a TFT substrate in which the conventional ladderstructure is adopted.

As shown in FIG. 1( a), on the TFT substrate 1, the two pixels A and Bare provided adjacent to each other in the direction parallel to thescanning lines, thereby forming a pair. The signal lines 12A and 12B,which respectively charge the pixels A and B, are both provided on thepixel electrode 5A of the pixel A. Therefore, in the row of the pixels Ato D, a pixel electrode 5 on which signal lines 12 are provided and apixel electrode 5 on which signal lines 12 are not provided are arrayedalternately in the direction parallel to the scanning lines (not shown).

As in the ladder structure, the signal lines 12A and 12B are providedbetween both edges of the pixel electrode 5A, the both edges beingparallel to the signal lines 12. The signal lines 12A and 12B aredistanced from each other to such a degree as to make up for the shift.Likewise, the signal lines 12C and 12D are provided between both edgesof the pixel electrode 5C, the both edges being parallel to the signallines 12. The signal lines 12C and 12D are distanced from each other asto make up for the shift.

FIG. 4 is a plan view illustrating the TFT substrate 1. In FIG. 4, theplurality of lines extending in the lateral direction are scanning lines14. The plurality of lines intersecting the scanning lines 14 are thesignal lines 12. The rectangular members indicated by virtual lines arethe pixel electrodes 5. The pixel electrodes 5 are positioned so thatperipheral parts thereof are superimposed onto the scanning lines 14 andthe signal lines 12. The scanning lines 14, the signal lines 12, and thepixel electrodes 5 are provided in this order on the substrate 4 shownin FIG. 2 or FIG. 3. The substrate 4 is made of such material as glass.Between an electrode layer including the scanning line 14 and anelectrode layer including the signal lines 12, a gate insulating film(not shown) is provided. Between the electrode layer including thesignal lines 12 and an electrode layer including the pixel electrodes 5,an interlayer insulating film (not shown) is provided. In FIG. 4, themembers indicated by the reference numeral 13 are TFTs (active elements)for supplying signals to the pixel electrodes 5, the signals beingapplied to the signal lines 12.

As explained above with reference to FIG. 1( a), each pair of signallines 12 are provided on either one of two pixel electrodes 5 that forma pair in the direction parallel to the scanning lines 14.

In FIG. 4, the member indicated by reference numeral 15 is a Cs wire,and the member indicated by reference numeral 16 is a Cs electrode. TheCs wire and the Cs electrode provide a storage capacitance. The Cs wireis provided between each pair of the scanning electrodes 14. The Cs wireis provided in the same electrode layer with the scanning lines 14. Thestorage capacitance is provided at an superimposed portion of the Cswire 15 and the Cs electrode 16, by the Cs wire 15, the Cs electrode 16,and the gate insulating film provided therebetween. The member 17, whichis provided to each pixel and indicated by chain line, is a contact holefor connecting the Cs electrode 16 and the pixel electrode 5.

As shown in FIG. 4, on the TFT substrate 1, in the direction parallel tothe scanning lines 14, the pixel electrode 5 is larger in the pixels Aand C (in which the signal lines 12 are provided intensively) than inthe pixels B and D (in which the signal lines 12 are not provided). Thisarrangement is adopted so as to equalize the aperture area in the pixelA and the aperture area in the pixel B. By thus equalizing the apertureareas, an excellent white balance is attained.

Next, capacitances Csd provided in each pixel according to the foregoingarrangement is described, with reference to FIGS. 5( a) and 5(b), FIGS.6( a) and 6(b) (cross-sectional views of FIGS. 5( a) and 5(b)), FIGS. 7(a) and 7(b), and FIGS. 8( a) and 8(b) (cross-sectional views of FIGS. 7(a) and 7(b)).

First, with reference to FIGS. 5( a) and 5(b), and FIGS. 6( a) and 6(b)(cross-sectional views of FIGS. 5( a) and 5(b)), the capacitances Csdprovided in the pixel A, on which two signal lines 12 are provided, isdescribed (the following description is also applicable to the pixel C).

In the pixel A, the signal line 12A and the signal line 12B areprovided. Therefore, as shown in FIGS. 5( a) and 6(a), at those portionswhere the pixel electrode 5A is superimposed through the interlayerinsulating film (not shown) onto the signal lines 12A and 12B,capacitances CsdA and CsdB are provided, respectively. As describedabove, there is always a signal flowing on the signal line 12A, even inan OFF-period of the TFT 13A. Therefore, the drain potential of the TFT13A fluctuates through the capacitances CsdA and CsdB, as the potentialof the signal line 12A fluctuates. As described above, the fluctuationΔVdr caused by the fluctuation of the potential on the signal lines 12Aand 12B is represented by the following formula:ΔVdr=CsdA/Cpix×ΔVsA+CsdB/Cpix×ΔVsBwhere CsdA is a capacitance provided by the signal line 51A (whichcharges the drain electrode of the TFT 13A), the pixel electrode 5A, andthe interlayer insulating film; CsdB is a capacitance provided by theadjacent signal line 12B, the pixel electrode 5A, and the interlayerinsulating film; Cpix is a sum of capacitances associated with thedrain; ΔVsA is a value of voltage fluctuation obtained by subtracting apre-change potential of the signal line 12A from a post-change potentialof the signal line 12A; and ΔVsB is a value of voltage fluctuationobtained by subtracting a pre-change potential of the adjacent signalline 12B from a post-change potential of the adjacent signal line 12B.

As described above, the liquid crystal display devices 20 and 21 aredriven by the dot inversion driving. Therefore, if the polarity of asignal flowing on the signal line 12A is positive, the polarity of asignal flowing on the signal line 12B is negative. As a result, theinfluences on the drain potential are opposite, and cancel out eachother.

In the pixel A, the signal lines 12A and 12B are distanced from eachother, and provided between both edges of the pixel electrode 5A, theboth edges being parallel to the signal lines 12. With this arrangement,as shown in FIGS. 5( b) and 6(b), the area of the superimposed portionsdoes not change, even if the pixel electrode is shifted with respect tothe signal lines. Therefore, even if the pixel electrode is shifted withrespect to the signal lines, the capacitances CsdA and CsdB will not beinfluenced significantly.

Next, with reference to FIGS. 7( a) and 7(b), and FIGS. 8( a) and 8(b)(cross-sectional views of FIGS. 7( a) and 7(b)), the capacitances Csdprovided in the pixel B, on which signal lines 12 are not provided, isdescribed (the following description is also applicable to the pixel D).

In the pixel B, the signal lines 12 are not provided. Therefore, thereis no overlap between the signal lines 12 and the pixel electrode 5B.However, as shown in FIGS. 7( a) and 8(a), due to the influence of anelectric field in an oblique direction, a capacitance CsdB′ is providedbetween the signal line 12B and the pixel electrode 5B. Likewise, due tothe influence of an electric field in an oblique direction, acapacitance CsdC′ is provided between the signal line 12C and the pixelelectrode 5B. The signal line 12C is provided in the pixel C, which isthe adjacent pixel on the opposite side of the pixel A. The signal line12C supplies a signal to the pixel C. The capacitances CsdB′ and CsdC′are small, i.e. respectively not higher than one-quarter of thecapacitances CsdA and CsdB, which are formed in the pixel A. Inaddition, the polarity of a signal flowing on the signal line 12B andthe polarity of a signal flowing on the signal line 12A are opposite.Therefore, the influences on the drain potential of the pixel B cancelout each other.

As shown in FIGS. 7( b) and 8(b), because the pixel electrode 5B isdistanced from the signal lines 12B and 12C, the capacitances CsdB′ andCsdC′ will not change significantly, even if the pixel electrode 5 isshifted with respect to the signal lines 12.

If the TFT substrate 1 is arranged as described above, and used in aliquid crystal device driven by the dot inversion driving, thefluctuation of the drain potential that occurs through the capacitanceCsdA (which is formed at one superimposed portion) and the fluctuationof the drain potential that occurs through the capacitor CsdB (which isformed at the other superimposed portion) are opposite in the pixel A(in which two signal lines are provided) during an OFF-period of thethin-film transistor. Thus, like the ladder structure, the foregoingarrangement improves display quality, because the fluctuation of thedrain potential caused by the capacitors Csd (CsdA and CsdB) is small.

The two signal lines 12 are provided between the edges of the pixelelectrode 5, the edges being parallel to the signal lines. With thisarrangement, the area of the superimposed portions of the signal lines12 and the pixel electrode 5 will not change, even if the pixelelectrode 5 is shifted with respect to the signal lines 12. Therefore,no significant change occurs in the capacitances provided at those partswhere the two signal lines 12 are provided. Thus, like the ladderstructure, the foregoing arrangement makes it possible to set a wideprocess margin.

On the other hand, in the pixel B, in which no signal line is provided,the capacitances (CsdB′ and CsdC′) are respectively provided by theoblique electric fields between the pixel electrode 5 and the signalline 12 in the pixel A, and between the pixel electrode 5 and the signalline 12 in the pixel C. However, the polarity of the signal supplied tothe signal line 12 in the pixel A and the polarity of the signalsupplied to the signal line 12 in the pixel C are opposite. Therefore,the influences on the drain potential exerted through the capacitancescancel out each other.

Even if the pixel electrode 5 is shifted with respect to the signallines 12, the influence of the shift on the capacitances, which arerespectively formed by the oblique electric fields, is too small to be aproblem, because the pixel electrode 5 is distanced from the signallines 12.

The foregoing arrangement is simpler than the conventional ladderstructure, because the signal lines 12 of each pixel are not divided. Inaddition, the foregoing arrangement reduces the area occupied by thesignal lines 12 with respect to the area of apertures. Therefore, theaperture ratio of the panel as a whole is improved.

Especially according to the foregoing arrangement, as shown in FIG. 1(a), the pixel on which the signal lines 12 are provided and the pixel onwhich no signal line is provided are arrayed alternately. Therefore, thecapacitances provided by the oblique electric fields between the pixelelectrode 5 and the signal lines 12 in the adjacent pixels can becanceled out effectively. In a pixel in which the signal lines 12 arenot provided, the capacitances (CsdB′ and CsdC′) are provided by theoblique electric fields between (i) the pixel electrode of that pixeland (ii) the signal lines 12 respectively provided on the pixelelectrodes 5 of adjacent pixels. By alternately arraying a pixel onwhich the signal lines 12 are provided and a pixel on which the signallines 12 are not provided, it is possible to equalize values of thecapacitances provided by the oblique electric fields at both sides ofthe pixel electrode on which the signal lines 12 are not provided. As aresult, display quality is further improved.

In addition, with the foregoing arrangement, the directions of outgoinglines from the TFTs 13 to the contact holes 17 can be unified. That is,the directions of the TFTs 13 can be unified. Therefore, it is possibleto minimize the influence of shift on the display quality at activeelement portions.

FIG. 9 is a plan view illustrating a TFT substrate 1 of a secondembodiment of the present invention. For the purpose of explanation,members whose functions are identical to those of the members describedin the first embodiment are labeled with identical reference numerals,and explanations for such members are omitted.

On the TFT substrate 1 of the present embodiment, two signal lines areprovided in the middle of a pixel on which two signal lines areprovided. That is, in a pixel A, a signal line 12A and a signal line 12Bare provided in the middle. On the TFT substrate 1 of FIG. 4, the Cselectrode 16, which provides the storage capacitance, is providedbetween the signal line 12A and the signal line 12B. On the other hand,on the TFT substrate 1 of the present embodiment, Cs electrodes 16A-1and 16A-2 are respectively provided at two positions: one is between thesignal line 12A and one edge of the pixel electrode 5A, and the other isbetween the signal line 12B and the other edge of the pixel electrode5A.

By thus providing the signal lines 12A and 12B in the middle of thepixel A, the aperture ratio of the liquid crystal device 20 or 21 as awhole is lower than that of the liquid crystal device 20 or 21 using theTFT substrate 1 of FIG. 4. However, because the distance between thesignal line 12B and the pixel electrode 5B and the distance between thesignal line 12C and the pixel electrode 5B are longer, the values of thecapacitances CsdB′ and CsdC′ (which are provided by the oblique electricfields between the pixel electrode 5B and the signal line 12B, andbetween the pixel electrode SB and the signal line 12C, respectively)are reduced to approximately one-tenth of the values in the structure ofFIG. 4.

According to the TFT substrate 1 of the present embodiment, in thedirection parallel to the scanning lines 14, the pixel electrode 5 islarger in the pixels A and C (on which the signal lines 12 are notprovided) than in the pixels B and D (on which the signal lines 12 areprovided intensively). This arrangement is adopted in order to equalizethe aperture area of each pixel.

FIG. 10 is a plan view illustrating a TFT substrate 1 of a thirdembodiment of the present invention. For the purpose of explanation,members whose functions are identical to those of the members describedin the first embodiment or the second embodiment are labeled withidentical reference numerals, and explanations for such members areomitted.

According to the TFT substrate 1 of the present embodiment, the signalline 12B, which charges the pixel B, has two portions. One is providedon the corresponding pixel electrode 5B. The other is a roundaboutportion provided on the pixel electrode 5A, which is adjacent to thepixel electrode 5B in the direction parallel to the scanning lines 14.Likewise, the signal line 12C, which charges the pixel C, has twoportions. One is provided on the corresponding pixel electrode 5C. Theother is a roundabout portion provided on the pixel electrode 5B, whichis adjacent to the pixel electrode 5C in the direction parallel to thescanning lines 14. The signal lines 12 a and 12D, which respectivelycorrespond to the pixels A and D, are arranged in the like manner. Oneach pixel electrode 5, a portion of the corresponding signal line 12and a roundabout portion of the signal line 12 of the adjacent pixelelectrode 5 are provided as a pair. Here, attention is focused on thepixel B. On the pixel electrode SB, a portion of the signal line 12B anda roundabout portion of the signal line 12C, which corresponds to theadjacent pixel C, are provided as a pair. Likewise, attention is focusedon the pixel C. On the pixel electrode 5C, a portion of the signal line12C and a roundabout portion of the signal line 12D, which correspondsto the adjacent pixel D, are provided as a pair. Except those portionsthat lie between pixel electrodes 5, the signal lines 12 are providedbetween the edges of the pixel electrode 5 that is superimposed on thesignal lines 12.

According to this arrangement, a pixel electrode 5 has a portion onwhich two signal lines 12 (including a signal line corresponding to anadjacent pixel) are provided, and a portion on which the signal lines 12are not provided at all. Here again, except those portions that liebetween pixel electrodes 5, the signal lines 12 are provided between theedges of the pixel electrode 5. Therefore, even if the pixel electrode 5is shifted with respect to the signal lines 12, the superimposed area ofthe signal lines 12 and the pixel electrode 5 will not change. If ashift occurs, the shift changes the capacitances Csd at those portionsof the signal lines 12 that lie between pixel electrodes. However,because most portions of the signal lines 12 are covered with the pixelelectrodes 5, the change is too small to be a problem.

Therefore, as in the cases of the TFT substrate 1 of the firstembodiment (see FIG. 4) and the second embodiment (see FIG. 9), if theTFT substrate 1 of the present embodiment is used in a liquid crystaldisplay device that is driven by the dot inversion driving, the liquidcrystal device can reduce the drain potential fluctuation at each pixel(the drain potential fluctuation occurs during an OFF-period of thethin-film transistor through the capacitances CsdA and CsdB, which arerespectively provided at the superimposed portions). As a result,display quality is improved.

Moreover, the foregoing arrangement is simpler than the conventionalladder structure, because the signal lines 12 of each pixel are notdivided. In addition, the aperture ratio of the panel as a whole isimproved, because the area occupied by the signal lines 12 with respectto the area of apertures is reduced.

The structure of the TFT substrate 1 of the present embodiment isadvantageous also in that the signal lines 12 are allocated to eachpixel evenly in terms of the area occupied by the signal lines 12.Again, if the area occupied by the signal lines 12 is different frompixel to pixel, the aperture area can be equalized by adopting thefollowing arrangement: in the direction parallel to the scanning lines,a pixel electrode 5 in which a large area is occupied by the roundaboutportion of a signal line 12 has a larger size than a pixel electrode inwhich a small area is occupied by the roundabout portion of a signalline 12. As a result, an excellent white balance is be attained.

Although not shown in FIGS. 4, 9, and 10, each TFT substrate 1 has alight-shielding pattern section that covers (i) gaps between pixelelectrodes 5 and (ii) the TFTs 13. The light-shielding pattern sectionprevents light from causing malfunction of the thin-film transistors,and ensures that no light is transmitted through the gaps.

Next, a fourth embodiment of the present invention is described. For thepurpose of explanation, members whose functions are identical to thoseof the members described in the first to third embodiments are labeledwith identical reference numerals, and explanations for such members areomitted.

In the first embodiment, as shown in the TFT substrate 1 of FIG. 4, thesize of the pixel electrode 5 in the direction parallel to the scanninglines 14 is larger in the pixels A and C (in which the signal lines 12are provided intensively) than in the pixels B and D (in which thesignal lines 12 are not provided). This arrangement is adopted so as toequalize the aperture area of the pixel A and the aperture area of thepixel B.

In the present embodiment, the foregoing arrangement is modified so thatthe aperture area of the pixels B and D (in which the signal lines 12are not provided) is one-half of the aperture area of the pixels A and C(in which the signal lines 12 are provided intensively). In other words,the TFT substrate 1 of the present embodiment is arranged so that apixel electrode corresponding to a pixel in which the signal line 12 arenot provided has a smaller size in the direction parallel to thescanning lines 14 than a pixel electrode corresponding to a pixel inwhich the signal lines 12 are provided intensively. This arrangement isadopted so that the aperture area of the pixel in which the signal lines12 are not provided is one-half of the aperture area of the pixel inwhich the signal lines 12 are provided intensively.

With this arrangement, the TFT substrate 1 of the present embodiment canbe used suitably in a liquid crystal 20 or 21 including a color filter 7shown in FIG. 11. With reference to FIGS. 11 and 12, the followingspecifically describes the present embodiment.

FIG. 11 is a schematic diagram illustrating a color arrangement of thecolor filter 7.

The color filter 7 includes a plurality of filters of three primarycolors: red (R), green (G), and blue (B). In the color filter 7, filtersof B (first), G (second), R (third), and G (fourth) constitute one colorfilter unit, and a plurality of such blocks are arrayed.

For example, in the direction parallel to the signal lines 12 shown inFIG. 4, the color filter 7 includes columns in which G-filters arearrayed successively, and columns in which a B-filter and an R-filterare arrayed alternately.

The color filter described in the present embodiment is arranged so thatthe second and fourth filters in each block are G-filters. However, thepresent invention is not limited to this arrangement. If the pixelelectrodes corresponding to the first and third filters have half thesize of the pixel electrodes corresponding to the second and fourthfilters, the first and the third filters may be G-filters.

The area of a G-filter is one-half of the area of an R-filter or aB-filter. Therefore, in the color filter 7, each color has the sametotal area.

Described next is a case in which the TFT substrate is used in a liquidcrystal display device 20 or 21 including the color filter 7 having theforegoing color arrangement.

FIG. 12 is a schematic plan view illustrating a liquid crystal displaydevice including the TFT substrate and the color filter 7. For thepurpose of explanation, only the color filter 7, the pixel electrodes 5of the TFT substrate, and the signal lines 12 of the TFT substrate areshown in FIG. 12.

As shown in FIG. 12, the filters of B, G, and R respectively correspondto the pixel electrodes 5 provided on the TFT substrate 1, andrespectively form a B-pixel, a G-pixel, and an R-pixel.

Specifically, as shown in FIG. 12, the signal lines 12 are not providedon the pixel electrodes 5B, 5D, 5F, and 5H, which are respectivelyopposed to G-filters. The signal lines 12A and 12B are providedintensively on the pixel electrode 5C, which is opposed to a B-filter.The signal lines 12C and 12D are provided intensively on the pixelelectrode 5C, which is opposed to an R-filter. The signal lines 12E and12F are provided intensively on the pixel electrode 5E, which is opposedto an e-filter. The signal lines 12G and 12H are provided intensively onthe pixel electrode 5G, which is opposed to an R-filter.

As described above, in the TFT substrate of the present embodiment, apixel electrode corresponding to a pixel on which the signal lines 12are not provided has a smaller size in the direction parallel to thescanning lines 14 than a pixel electrode corresponding to a pixel onwhich the signal lines 12 are provided intensively. This arrangement isadopted so that the aperture area of the pixel on which the signal lines12 are not provided is one-half of the aperture area of the pixel onwhich the signal lines 12 are provided intensively. Therefore, in FIG.12, the pixel electrodes 5B, 5D, 5F, and 5H, which respectivelycorrespond to G-pixels (pixels on which the signal line 12 are notprovided) have a smaller size in the direction parallel to the scanninglines 14 than the pixel electrodes 5A, 5C, 5E, and 5G, whichrespectively correspond to R-pixels or B-pixels (pixels on which thesignal lines 12 are provided intensively).

With this arrangement, the aperture area in each G-pixel is one-half ofthe aperture area of an R-pixel or a B-pixel.

Therefore, because the liquid crystal display device of the presentembodiment includes the color filter 7 having the foregoing colorarrangement, the number of green (G) pixels, which have a high level ofvisibility for human, is twice the number of red pixels and blue pixels.Therefore, higher resolution can be attained as compared with a liquidcrystal display device in which the same number of pixels are providedfor each color (see, for example, Japanese Publication for PatentApplication, Tokukouhei 03-36239 (publication date: May 30, 1991;corresponding to U.S. Pat. No. 4,642,619)).

In the liquid crystal display device of the present embodiment, althoughthe number of G-pixels are twice the number of R-pixels or B-pixels, theaperture area of a G-pixel is one-half of the aperture area of anR-pixel or a B-pixel. Therefore, the total aperture area is the sameamong the pixels of B, G, and R. As a result, an excellent white balanceis attained.

By using the TFT substrate of the foregoing arrangement, the structureof the G-pixels, which are fine, is simplified, because the signal lines12 are not provided on a G-pixel, whose aperture area is one-half of theaperture area of an R-pixel or a B-pixel. Therefore, it is possible toprevent the decrease of yield from occurring due to microfabrication andhigh-density packaging of wires.

As described above, the present invention is suitable not only for acolor filter having an odd-number cycle (in which R, G, and B constituteone block), but also for a color filter having an even-number cycle (inwhich filters of the same color are used in one color unit, e.g. RGBG).

In the present embodiment, the liquid crystal display device includes aTFT substrate in which the signal lines 12 are arranged in the samemanner as the signal lines 12 on the TFT substrate 1 of the firstembodiment. However, the color filter 7 can be used together with a TFTsubstrate in which the signal lines 12 are arranged in the same manneras the signal lines 12 on the TFT substrate 1 of the second embodimentsee FIG. 9), provided that the ratio between the aperture area of apixel on which the signal lines 12 are not provided and the aperturearea of a pixel on which the signal lines 12 are provided intensively is1:2. This condition can be satisfied by designing the pixel electrodesso that the pixel electrode corresponding to the pixel on which thesignal lines 12 are not provided has a smaller size in the directionparallel to the scanning lines 14 than the pixel electrode correspondingto the pixel on which the signal lines 12 are provided integrally.

To attain the foregoing object, a first active element substrate of thepresent invention includes a plurality of signal lines; a plurality ofscanning lines intersecting the signal lines; an active element providedat each intersection between the signal lines and the scanning lines;and a pixel electrode provided at each intersection between the signallines and the scanning lines, the pixel electrode being superimposed atleast on the signal lines, wherein: signal lines respectivelycorresponding to a pair of pixel electrodes are provided intensively onone of the pair of pixel electrodes so as to be located between edges ofsaid one of the pair of pixel electrodes, the pair of pixel electrodesbeing adjacent to each other in a direction parallel to the scanninglines. Therefore, there is an effect that it is possible to provide anactive element substrate that can (i) reduce, while ensuring a wideprocess margin, the fluctuation of the potential of a terminal to beconnected to a pixel electrode (the fluctuation of the potential occursduring OFF-period of an active element due to capacitances respectivelyprovided by superimposing a pixel electrode on signal lines), (ii)simplify the arrangement of signal lines, and (iii) improve the apertureratio.

According to this arrangement, the signal lines respectivelycorresponding to the pair of pixel electrodes are provided only on oneof the pair of pixel electrodes. Therefore, on a pixel electrode, twosignal lines (including a signal line corresponding to an adjacent pixelelectrode) are provided, or no signal line is provided at all.

If such an active element substrate is used in the liquid crystaldisplay device that is driven by the dot inversion driving, in a pixelof a pixel electrode on which two signal lines are provided,fluctuations of the drain (an example of the terminal connected to thepixel electrode) potential occurs in opposite directions, thefluctuation occurring during an OFF-period of a thin-film transistor (anexample of an active element) through the capacitances Csd1 and Csd2respectively provided at superimposed portions. Therefore, as in theladder structure described above, it is possible to reduce thefluctuation of the drain potential caused by the capacitances Csd (Csd1and Csd2), thereby improving display quality.

In addition, the two signal lines provided on a pixel electrode arelocated between edges of the pixel electrode, the edges being parallelto the signal lines. Therefore, even if the pixel electrode is shiftedwith respect to the signal lines, the area of the superimposed portionsof the signal lines and the pixel electrode does not change. As aresult, no significant change occurs in the capacitances provided at thepositions where the two signal lines are respectively provided. Thismakes it possible to set a wide process margin.

On the other hand, in a pixel in which no signal line is provided on apixel electrode, if signal lines are provided in adjacent pixels,capacitances are respectively provided by oblique electric fieldsbetween the signal lines and the pixel electrode. However, the twosignal lines that respectively provide the capacitances by means of theoblique electric fields are supplied with signals of oppositepolarities, respectively. Therefore, the influences exerted through thecapacitances on the drain potential cancel out each other.

Even if the pixel electrode is shifted with respect to the signal lines,the influence of the shift on the capacitances respectively provided bythe oblique electric fields are too small to be a problem. This isbecause the pixel electrode and the signal lines are distanced (notsuperimposed).

Such an arrangement is simpler than the conventional ladder structure,because the signal lines of each pixel are not divided. In addition, theaperture ratio of the panel as a whole is improved, because the areaoccupied by the signal lines with respect to the area of apertures isreduced. This is particularly suitable for a high-definition liquidcrystal display device having a short pixel pitch.

Note that a signal line corresponding to a pixel electrode means asignal line that charges a pixel including the pixel electrode. From theside of the signal line, the pixel electrode charged by the signal linecan be expressed as a corresponding signal electrode. Also note that,although such expressions as “signal lines are provided on a pixelelectrode” is used in order to describe the superimposition of thesignal lines and pixel electrode, this does not mean a hierarchicalrelationship on the substrate among the signal lines, scanning lines,and pixel electrodes. To provide easy-to-understand explanation, TFTsare used as an example of active elements, and effects of the presentinvention are described using the phenomena caused in the case of TFTs.

To attain the foregoing object, in the first active element substrate ofthe present invention, a pixel electrode on which signal lines areprovided and a pixel electrode on which no signal line is provided arearrayed alternately in the direction parallel to the scanning lines.Therefore, as compared with an arrangement in which these pixelelectrodes are not arrayed alternately, it is possible to moreeffectively cancel out the capacitances respectively provided by theoblique electric fields between (i) a pixel electrode and (ii) signallines in the adjacent pixels.

In a pixel of a pixel electrode on which no signal line is provided,capacitances are respectively provided between (i) the pixel electrodeand (ii) signal lines provided on pixel electrodes of adjacent pixels.By alternately arraying, as described above, the pixel electrode onwhich signal lines are provided and the pixel electrode on which nosignal line is provided, it is possible to equalize the values of thecapacitances provided at both sides of the pixel electrode on which nosignal line is provided. Therefore, the capacitances can be canceled outeffectively. As a result, because the capacitances respectively providedby the oblique electric fields between (i) a pixel electrode and (ii)signal lines in the adjacent pixels can be canceled out more effectivelyas compared with an arrangement in which these pixel electrodes are notarrayed alternately, there is an effect that display quality is furtherimproved.

In addition, with the foregoing arrangement, the directions of outgoinglines from active elements to contact holes (for respectively connectingthe active elements with pixel electrodes) can be unified. That is, thedirections of the active elements can be unified. Therefore, there isalso an effect that it is possible to minimize the influence of shift onthe display quality at active element portions.

To attain the foregoing object, a second active element substrate of thepresent invention includes a plurality of signal lines; a plurality ofscanning lines intersecting the signal lines; an active element providedat each intersection between the signal lines and the scanning lines;and a pixel electrode provided at each intersection between the signallines and the scanning lines, the pixel electrode being superimposed atleast on the signal lines, wherein: each of the signal lines includes aportion provided on a corresponding pixel electrode and a roundaboutportion provided on a pixel electrode that is adjacent, in a directionparallel to the scanning lines, to said corresponding pixel electrode;and, on each pixel electrode, a portion of a corresponding signal lineand a roundabout portion of a signal line corresponding to an adjacentpixel electrode form a pair, and the pair is located between edges ofthe pixel electrode except those portions that lie between pixelelectrodes. Therefore, there is an effect that it is possible to providean active element substrate that can (i) reduce, while ensuring a wideprocess margin, the fluctuation of the potential of a terminal to beconnected to a pixel electrode (the fluctuation of the potential occursduring OFF-period of an active element due to capacitances respectivelyprovided by superimposing a pixel electrode on signal lines), (ii)simplify the arrangement of signal lines, and (iii) improve the apertureratio.

According to the foregoing arrangement, each signal line includes aroundabout portion, and, on each pixel electrode, a part of acorresponding signal line and a roundabout part of an adjacent signalline are provided as a pair. Therefore, in this case, each pixelelectrode includes a portion on which two signal lines are provided(including a signal line corresponding to an adjacent pixel electrode)and a portion on which no signal line is provided at all. Moreover, thetwo signal lines are located between edges of the pixel electrode,except those portions that lie between pixel electrodes. Therefore, thesuperimposed area of the signal lines and the pixel electrode does notchange, even if the pixel electrode is shifted with respect to thesignal lines. As a result, it is possible to reduce the change of valuesof capacitances provided where partial signal lines are provided as apair.

Like the first active element substrate described above, if such anactive element substrate is used, for example, in the liquid crystaldisplay device that is driven by the dot inversion driving, fluctuationsof the drain (an example of the terminal connected to the pixelelectrode) potential occurring in each pixel during an OFF-period of athin-film transistor (an example of an active element) through thecapacitances Csd1 and Csd2 respectively provided at superimposedportions can be reduced as in the ladder structure described above,while ensuring a wide process margin. Therefore, display quality isimproved. Moreover, the arrangement is simpler as compared with theconventional ladder structure, because the signal lines of each pixelare not divided. In addition, the area occupied by the signal lines withrespect to aperture portions is reduced. As a result, the aperture ratioof the panel as a whole is improved. In particular, the forgoingarrangement is advantageous in that the signal lines are allocated toeach pixel evenly in terms of the area occupied by the signal lines.

To attain the foregoing object, in the first active element substrateand the second active element substrate, a pixel electrode on whichsignal lines are provided intensively or a pixel electrode on which alarge area is occupied by a roundabout portion of a signal line has alarger size in the direction parallel to the scanning lines than a pixelelectrode on which no signal line is provided or a pixel electrode onwhich a small area is occupied by a roundabout portion of a signal line.Therefore, it is possible to equalize the aperture area of a pixel inwhich two signal lines are provided and a pixel in which no signal lineis provided. If a roundabout part of a signal line is provided, it ispossible to equalize the aperture area of pixels, and there is also aneffect that an excellent white balance is attained.

In the first active element substrate, an aperture area of a pixel of apixel electrode on which no signal line is provided is one-half of anaperture area of a pixel of a pixel electrode on which signal lines areprovided intensively. Therefore, the active element substrate of thepresent invention can be applied suitably to a liquid crystal displaydevice using a color filter in which filters of red, green, blue, andgreen (RGBG) constitute one block, for example.

A liquid crystal display device of the present invention includes: theactive element substrate of the present invention; an opposed substrateon which a common electrode is provided; and a liquid crystal layersandwiched between the active element substrate and the opposedsubstrate, a polarity of a voltage applied to one of two adjacent signallines being opposite a polarity of a voltage applied to the other of thetwo adjacent signal lines. Therefore, as described above, there is aneffect that it is possible to provide an active element substrate thatcan (i) reduce, while ensuring a wide process margin, the fluctuation ofthe potential of a terminal to be connected to a pixel electrode (thefluctuation of the potential occurs during OFF-period of an activeelement due to capacitances respectively provided by superimposing apixel electrode on signal lines), (ii) simplify the arrangement ofsignal lines, and (iii) improve the aperture ratio.

A liquid crystal display device of the present invention includes: anactive element substrate in which an aperture area of a pixel of a pixelelectrode on which no signal line is provided is one-half of an aperturearea of a pixel of a pixel electrode on which signal lines are providedintensively; an opposed substrate on which a common electrode isprovided; a liquid crystal layer sandwiched between the active elementsubstrate and the opposed substrate; and color filters of red, green,and blue, provided so that four adjacent pixel electrodes arrayed in thedirection parallel to the scanning lines constitute one color unit, thefour adjacent pixel electrodes consisting of two pixel electrodes onwhich signal lines are provided, and two pixel electrodes on which nosignal line is provided, the two pixel electrodes on which no signalline is provided respectively corresponding to color filters of green, apolarity of a voltage applied to one of two adjacent signal lines beingopposite a polarity of a voltage applied to the other of the twoadjacent signal lines. According to this arrangement, the number ofgreen pixels, which have a high level of visibility for human, is twicethe number of red and blue pixels. Therefore, higher resolution can beattained as compared with a liquid crystal display device in which thesame number of pixels are provided for each color. In addition, becausea green filter is provided so as to be opposed to a pixel electrode onwhich no signal line is provided, the total aperture area is the sameamong the pixels of red, blue, and green, even if the number of greenpixels is twice the number of red pixels and blue pixels. As a result,an excellent white balance is attained.

Moreover, the liquid crystal display device of the present inventionfurther includes a light-shielding pattern section provided on theactive element substrate or on the opposed substrate, thelight-shielding pattern section preventing light from being transmittedthrough gaps between pixel electrodes or through the active element.Therefore, it is possible to prevent light from being transmittedthrough a thin-film gap portion. According to the conventionalarrangement in which signal lines are provided so as to cover gapsbetween pixel electrodes, the signal lines themselves function aslight-shielding patterns. Thin-film transistors (active elements)malfunction when subjected to light. Therefore, by thus providing thelight-shielding pattern, there is an effect that the malfunctionoccurring due to light is prevented.

In the liquid crystal display device of the present invention, anoperation mode is a TN mode or an MVA mode.

In other words, the present invention can be expressed as follows.Namely, it is not the case that, in a liquid crystal display device, asignal line is provided so as to be covered at the same time with apixel electrode of a pixel and with a pixel electrode of an adjacentpixel. Instead, the signal line is covered completely with either one ofthe electrodes, except the case in which the signal line has a portionthat lies between pixel electrodes.

In this case, the liquid crystal display device may be arranged so thata pixel in which two signal lines are provided and a pixel in which nosignal line is provided are arrayed alternately.

In the liquid crystal display device, a signal line is, except in thecase in which the signal line bridges pixels, arranged so as to becompletely covered with either one of two pixels provided next to eachother in the direction of the scanning lines. However, a signal line maybe arranged so as to be provided not only on one of the two pixels, butalso partially provided on the other of the two pixels.

The liquid crystal display device may be arranged so that a polarity ofa voltage applied to one of two adjacent signal lines is opposite apolarity of a voltage applied to the other of the two adjacent signallines; and a pixel pitch in the direction of the scanning lines variesfrom pixel to pixel.

The liquid crystal display device may be a TN-type liquid crystaldisplay device including a liquid crystal layer aligned substantiallyhorizontally with respect to substrate surfaces, and twistedsubstantially by 90 degrees between the upper and lower substrates, theliquid crystal layer including a liquid crystal material having apositive dielectric anisotropy. Alternatively, the liquid crystaldisplay device may be an MVA-type liquid crystal display device,including a vertically aligned liquid crystal layer, the liquid crystallayer including a nematic liquid crystal material having a negativedielectric anisotropy.

In addition, the liquid crystal display device may further include alight-shielding pattern (BM) at gaps between pixel electrodes and at TFTparts, on the TFT substrate.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. An active element substrate, comprising: a plurality of signal lines;a plurality of scanning lines intersecting the signal lines; an activeelement provided at each intersection between the signal lines and thescanning lines; and a pixel electrode provided at each intersectionbetween the signal lines and the scanning lines, the pixel electrodebeing superimposed at least on the signal lines, wherein: signal linesrespectively corresponding to a pair of pixel electrodes are providedintensively on one of the pair of pixel electrodes so as to be locatedbetween edges of said one of the pair of pixel electrodes, the pair ofpixel electrodes being adjacent to each other in a direction parallel tothe scanning lines.
 2. The active element substrate as set forth inclaim 1, wherein: a pixel electrode on which signal lines are providedand a pixel electrode on which no signal line is provided are arrayedalternately in the direction parallel to the scanning lines.
 3. Theactive element substrate as set forth in claim 1, wherein: a pixelelectrode on which no signal line is provided has a larger size in thedirection parallel to the scanning lines than a pixel electrode on whichsignal lines are provided intensively.
 4. The active element substrateas set forth in claim 2, wherein: an aperture area of a pixel of a pixelelectrode on which no signal line is provided is one-half of an aperturearea of a pixel of a pixel electrode on which signal lines are providedintensively.
 5. A liquid crystal display device, comprising: an activeelement substrate including a plurality of signal lines, a plurality ofscanning lines intersecting the signal lines, an active element providedat each intersection between the signal lines and the scanning lines,and a pixel electrode provided at each intersection between the signallines and the scanning lines, the pixel electrode being superimposed atleast on the signal lines, wherein signal lines respectivelycorresponding to a pair of pixel electrodes are provided intensively onone of the pair of pixel electrodes so as to be located between edges ofsaid one of the pair of pixel electrodes, the pair of pixel electrodesbeing adjacent to each other in a direction parallel to the scanninglines; an opposed substrate on which a common electrode is provided; anda liquid crystal layer sandwiched between the active element substrateand the opposed substrate, a polarity of a voltage applied to one of twoadjacent signal lines being opposite a polarity of a voltage applied tothe other of the two adjacent signal lines.
 6. A liquid crystal displaydevice as set forth in claim 5, further comprising: a light-shieldingpattern section provided on the active element substrate or on theopposed substrate, the light-shielding pattern section preventing lightfrom being transmitted through gaps between pixel electrodes or throughthe active element.
 7. The liquid crystal display device as set forth inclaim 5, wherein: an operation mode is a TN mode.
 8. The liquid crystaldisplay device as set forth in claim 5, wherein: an operation mode is anMVA mode.
 9. A liquid crystal display device, comprising: an activeelement substrate including a plurality of signal lines, a plurality ofscanning lines intersecting the signal lines, an active element providedat each intersection between the signal lines and the scanning lines,and a pixel electrode provided at each intersection between the signallines and the scanning lines, the pixel electrode being superimposed atleast on the signal lines, wherein signal lines respectivelycorresponding to a pair of pixel electrodes are provided intensively onone of the pair of pixel electrodes so as to be located between edges ofsaid one of the pair of pixel electrodes, the pair of pixel electrodesbeing adjacent to each other in a direction parallel to the scanninglines; a pixel electrode on which signal lines are provided and a pixelelectrode on which no signal line is provided are arrayed alternately inthe direction parallel to the scanning lines; and an aperture area of apixel of a pixel electrode on which no signal line is provided isone-half of an aperture area of a pixel of a pixel electrode on whichsignal lines are provided intensively; an opposed substrate on which acommon electrode is provided; a liquid crystal layer sandwiched betweenthe active element substrate and the opposed substrate; and colorfilters of red, green, and blue, provided so that four adjacent pixelelectrodes arrayed in the direction parallel to the scanning linesconstitute one color unit, the four adjacent pixel electrodes consistingof two pixel electrodes on which signal lines are provided, and twopixel electrodes on which no signal line is provided, the two pixelelectrodes on which no signal line is provided respectivelycorresponding to color filters of green, a polarity of a voltage appliedto one of two adjacent signal lines being opposite a polarity of avoltage applied to the other of the two adjacent signal lines.